1. Technical Background
The present invention relates generally to treating vascular defects, and more particularly, to a stent system and method for repair and treatment of blood vessels.
2. Discussion
On a worldwide basis, nearly one million balloon angioplasties were performed in 1997 to treat vascular disease, including blood vessels clogged or narrowed by a lesion or stenosis. The objective of this procedure is to increase the inner diameter or cross-sectional area of the vessel passage, or lumen, through which blood flows. Unfortunately, the lumen often closes or narrows again within six months after balloon angioplasty, a phenomenon called restenosis.
Another serious vascular defect is an area of weakened vessel wall that causes a bulge or bubble to protrude out in a radial direction from the adjacent vessel. This type of defect is called an aneurysm. If untreated, the aneurysm may continue expanding until it bursts, causing hemorrhage.
In an effort to prevent restenosis or treat an aneurysm without requiring surgery, short flexible cylinders or scaffolds made of metal or polymers are sometimes placed into a vessel to maintain or improve blood flow. Referred to as stents, various types of these devices are widely used for reinforcing diseased blood vessels, for opening occluded blood vessels, and occasionally for defining an internal lumen bulkhead to relieve pressure in an aneurysm. The stents allow blood to flow through the vessels at an improved rate while providing the desired lumen opening or structural integrity lost by the damaged vessels. Some stents are expanded to the proper size by inflating a balloon catheter, referred to as "balloon expandable" stents, while others are designed to elastically resist compression in a "self-expanding" manner.
Balloon expandable stents and self-expanding stents are generally delivered in a cylindrical form, crimped to a smaller diameter around some type of catheter-based delivery system. When positioned at a desired site within the lesion, they are expanded by a balloon or allowed to "self-expand" to the desired diameter. However, many vessels are too small to accept a stent shaped in a cylinder during delivery.
Another type of stent is formed of a wire that has a relaxed cylindrical shape, yet can be stretched into a linear shape for delivery through a much smaller catheter than any stent delivered in cylindrical form. The basic design of such a "linear" stent is described in U.S. Pat. No. 4,512,338, issued Apr. 23, 1985 to Balko, and is of course acceptable for certain applications.
Balko discloses a shape memory nitinol wire, shaped in its parent phase into a coil of adjacent wire loops, then cooled to its martensite phase and reshaped to a straight shape. The wire is inserted into the vessel with thermal insulation, such that the wire reforms to its coil shape upon the removal of the insulation means, so as to reform the damaged vessel lumen.
However, this basic linear stent often creates gaps between adjacent helical portions of wire in its deployed shape, gaps which may thrombose or restenose. Moreover, many aneurysms form at a bifurcation, where one vessel branches off from another, but the basic linear stent is generally ineffective treatment for such a bifurcation aneurysm.
As a result, there is a need for an improved stent that can be easily delivered to a vascular site through a very small catheter, that is capable of being atraumatically repositioned, and that exhibits sufficient structural integrity and resilience under inward forces. It is also desirable that this improved stent be designed to reduce the possibility of interstitial gaps, and it is preferable that the stent system be capable of effectively treating a bifurcation aneurysm.
The present invention provides an intravascular stent constructed from a resilient or superelastic material for holding open an occluded vessel passageway, or for providing supposed to a weakend vessel site such as an aneurysm. Preferably, the stent of the present invention can be delivered in a linear fashion through a small catheter, yet can expand into a relaxed cylindrical shape on deployment from the catheter. Moreover, the stent system of the present invention can effectively treat a bifurcation aneurysm, by providing a pair of meshed stents extending into the branches of a bifurcation, thus building a "shelf" for supporting embolic devices or materials in the aneurysm.
The stent is substantially helical in its relaxed state, formed of a spiral wire having a pitch of preferably about 0.125 inches. Its helical shape defines a passageway or lumen, and is inserted into the vessel near the damaged or occluded vessel site through a catheter smaller in diameter than the deployed stent itself. The stent can be stretched to a substantially linear shape for insertion within the lumen of a catheter. When released from the catheter into the vessel, the stent tends to assume a helical configuration, thereby expanding in diameter and maintaining its position at the vessel site, where it exerts a radially outward force tending to hold open the vessel.
In particular, the stent of the present invention exhibits a relaxed helical configuration that includes a proximal end defining a first stent passageway opening. A body portion extends from the proximal end and defines a passageway. A distal end of the stent terminates the body portion and defines a second passageway opening a predetermined axial distance from the first passageway opening.
These and various other objects, advantages and features of the invention will become apparent from the following description and claims, when considered in conjunction with the appended drawings.